1. Field of the Invention
The present invention relates to an electrode structure including an electrode and an electrode lead and a method of manufacturing the same, and a battery including an electrode structure and a method of manufacturing the same.
2. Description of the Related Art
In recent years, portable electronic devices such as camera-integrated VTRs (videotape recorders), cellular phones, or laptop computers are widely used, and size and weight reduction in the portable electronic devices and an increase in longevity of the portable electronic devices have been strongly demanded. Accordingly, as power sources for the portable electronic devices, the development of batteries, specifically lightweight secondary batteries capable of being used repeatedly at a high energy density have been promoted.
Among them, a secondary battery (a so-called lithium-ion secondary battery) using insertion and extraction of lithium for charge-discharge reaction holds great promise, because the secondary battery can obtain a large energy density, compared to a lead-acid battery or a nickel-cadmium battery.
The lithium-ion secondary battery includes a cathode structure including a cathode and a cathode lead attached to the cathode and an anode structure including an anode and an anode lead attached to the anode as electrode structures. The cathode and the anode both have a configuration in which an active material layer is arranged on a current collector. As a typical example of the lithium-ion secondary battery, a lithium-ion secondary battery using a lithium-containing complex oxide such as lithium cobalt oxide as a cathode active material and a carbon material as an anode active material is known, and these electrodes are formed by a so-called coating method. The coating method is a method of forming an active material layer by dissolving a powdery active material as well as a binder or the like in an organic solvent to form mixture slurry, and then coating a current collector with the mixture slurry, and drying the mixture slurry.
In the coating type electrode, when an electrode lead is bonded to an active material layer by welding, material properties of the electrode lead and the active material layer are not compatible, so there is a tendency that bonding strength is reduced, and the electrical resistance of a bonding part is increased. In this case, internal resistance is increased, so capacity characteristics or cycle characteristics easily decline.
Therefore, in the case where the coating type electrode is used, the current collector is intermittently coated with the mixture slurry to pattern-form the active material layer, so the current collector has a coated portion and an uncoated portion, thereby the electrode lead is bonded to an exposed surface of the uncoated portion of the current collector.
Recently, as portable electronic devices have become more sophisticated and multifunctional, a further increase in capacity has been demanded. Therefore, instead of a carbon material, it is considered to use silicon or the like as an anode active material. The theoretical capacity of silicon (4199 mAh/g) is much larger than the theoretical capacity of graphite (372 mAh/g), so a significant increase in capacity can be expected.
Moreover, in the case where the above-described silicon or the like is used as an active material, recently, it is considered to form an active material layer by a vapor-phase method, a liquid-phase method, a spraying method or the like instead of the coating method. By those methods, adhesion between the current collector and the active material layer is improved, and electrical conductivity between them is extremely increased, so an increase in capacity is expected.
However, in the case where a vapor-phase method or the like is used, the active material layer is formed on the whole surface of the current collector, so unlike the case where the coating method is used, it is difficult to pattern-form the active material layer. Therefore, to bond the electrode lead to the current collector, it is considered that after the active material layer is formed, a part of the active material layer is removed to expose the current collector. However, to remove the active material layer after forming the active material layer, a removing step is further necessary, so the number of steps is increased.
In addition to the above methods, to forcedly bond the electrode lead to the current collector without pattern-forming the active material layer, it is considered to use a typical bonding method such as a caulking method, a resistance welding method, an ultrasonic welding method or the like.
As a method of attaching the electrode lead to the electrode, some other methods are proposed. More specifically, to improve current collecting efficiency, productivity and yields, a method of attaching a lead plate for current collecting to an electrode plate, which is formed by filling a porous material mat with a paste-form active material, by caulking is known (for example, refer to Japanese Unexamined Patent Application Publication No. H02-223153). Moreover, to reduce the weight of a battery and obtain safety, a method in which a current collector is a layered body including a resin film and an electron-conductive thin film, and a lead is attached to the electron-conductive thin film is known (for example, refer to Japanese Unexamined Patent Application Publication No. H10-302753). In this case, the lead is fixed by a fixing means which does not cause dissolution of the electron-conductive thin film.